A custom list has numbered list, which refer style and the style refer numbered list. If the second numbered list is not imported before importing the first the import will fail. ArgumentException ("referenced document list does not exist."); is thrown and handled internally. It is caused by linked list styles. The style which refers other is imported before the referenced style. The RichTextBox expects that referred styles are imported before the styles that refer them. Available in R3 2018 Official Release Version.